A vendor will give you his taxpayer identification number, address and business name, and will certify he is not subject to backup withholding when he sends you a W-9 form, formally titled “Request for Taxpayer Identification Number and Certification.” The Internal Revenue Service requires that you obtain W-9 forms when you make inventory and service purchases from a vendor to create a paper trail that allows the agency to catch vendors who do not report income and to prevent you from claiming illegitimate vendor expenses.